Recovery of heavy oils by steam extraction



1955 F. F. CRAIG. JR.. ETAL Re. 25,913

RECOVERY OF HEAVY OILS BY STEAM EXTRACTION Original Filed Nov. 27, 1959TO STORAGE OR PROCESSING FF. CRAIG, JR. K L HUJSAK INVENTORS:

ATTORNEY Re. 25,918 Reissuecl Nov. 30, 1965 25,918 RECOVERY OF HEAVYOILS BY STEAM EXTRACTION Forrest F. Craig, Jr., and Karol L. Hujsak,both of Tulsa,

Okla., assignors to Pan American Petroleum Corporation, Tulsa, Okla., acorporation of Delaware Original No. 3,155,160, dated Nov. 3, 1964, Ser.No. 855,510, Nov. 27, 1959. Application for reissue Jan. 25, 1965, Ser.No. 443,735

10 Claims. (Cl. 166-40) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention relates to oil recovery and is directed particularly tothe recovery of heavy oils, tars, or bitumens from the undergroundstrata in which they occur. Specifically, it is directed, but notlimited to the in-place treatment and recovery of solid or semisolidhyr-docarbons, tars, or bitumens, one of the most notable examples ofwhich is the bitumen in the McMurray bituminous sand outcropping alongthe Athabasca River in Canada.

This invention is an improvement upon the invention of US. Patent2,881,838 for the recovery of heavy hydrocarbons by injecting steam intothe deposit and recovering the hydrocarbons therein which flow bygravity drainage to the base of a single well. It is a characteristic ofthese oils that they are substantially nonfiowable under reservoirconditions by the application of driving-fluid pressure. Furthermore,the reservoirs in which they occur generally lack any substantialsources of natural driving energy such as a natural water or gas drive.

The process described in that patent has been proved operable for therecovery of heavy oils, but for oils of the very high viscositycharacterizing the Athabasca tar, it is subject to the drawback that theforce of gravity drainage is able at best to produce only a small rateof flow into the well bore. This is because, even at the heat levelproduced by steam at an elevated pressure. the tar still has anappreciable viscosity which slows clown its flow. For example, even atthe 400 F. temperature of saturated steam at a pressure of 235 poundsper square inch gauge, the Athabasca tar viscosity is still about 8centipoises. Thus, even though tar with this viscosity can ultimately beproduced solely by gravity drainage, the time required to recover thetar which can be readily heated from a single well is so long thatsubstantial losses of heat to the overburden occur. Thus, thesubstantial fraction of the injected steam required to make up for theseheat losses constitutes an economic waste.

In view of the foregoing, it is a primary object of our invention toovercome the drawback of low producing rates in a process of the typedescribed, by supplementing the gravity-drainage driving forcesavailable to move the heated oil to the well bore. It is a furtherobject of the invention to provide a method of heavy-oil recoveryutilizing steam injection and gravity flow around a single well borewherein the flow of oil to the well bore is substantially augmented,loss of heat to the overburden is reduced, the ratio of steam injectedto oil recovered is lowered, and the time to carry the recoveryoperation to completion is reduced. Other and further objects, uses, andadvantages of the invention will become apparent as the descriptionproceeds.

Briefly stated, we have found by experimental studies that the foregoingobjects can be accomplished by a method which is a series of alternatingpressuring and depressuring steps, appropriately timed, wherein eachpressuring step comprises injecting steam into the formation whileholding back pressure and withdrawing liquids at about the same rate asthey accumulate at the well bore, exactly as disclosed and claimed insaid Patent 2,881,838. Each depressuring step, taken after a substantialbody of melted tar and steam condensate has accumulated within theformation, comprises releasing the back pressure and withdrawing fluidsfrom the bottom of the well as rapidly as possible to reduce thepressure there to a low value. This establishes a pressure gradientthroughout the heated volume of formation and, by revaporizing some ofthe condensate, thereby aids the force of gravity drainage in bringingthe melted tar and condensate to the well bore for recovery.

That is to say, each step or period of injecting steam at an elevatedtemperature and pressure into the formation as described in theafore-mentioned patent is continued for a period of time such that alarge body of oil will be contacted and have its temperature raised.Then the introduction of steam into the upper portion of the reservoiris discontinued, and the pressure at the well bore is reduced to as lowa value as possible. As a result of doing this at the time whensubstantial amounts of the semisolid tar have been heated and liquefied.and while a large amount of the steam condensate remains in thereservoir stratum, a pressure differential in the direction of the wellbore is created, which rapidly moves the accumulated condensate andmelted tar to the Well bore. The force thus provided is generallyproportional to the pressure at which steam was being injected prior tocut-ofi, and accordingly by utilizing steam at a sufficiently highpressure, it can be made may times as large as the gravity-drainageforce, which would otherwise be the only substantial force available forinducing flow into the well bore.

As the pressure at the well bore is reduced and more rapid flow towardthe well bore starts, the steam condensate is in part vaporized by theheat stored in the liquids and in the reservoir rock itself, so thatsome flashing of the condensate into steam occurs, adding to thegravity-drainage pressure to drive liquids toward the well bore. Ascompared with non-condensable gases. this flashing of the condensateinto steam by the stored heat in the rock and in the liquids maintainsthe pressure in a sense, so that it drops more slowly than if itresulted merely from the expansion of compressed gases. As a result ofthis depressuring step, the efficiency of the process is increasedseveral fold over that of a process utilizing gravity flow alone. Partof this increase in efficiency is due to the increased flow rate to thewell bore, and part is due to the shorter length of time required toproduce the oil recoverable from a single well bore, which therebyminimizes the heat loss to overlying formations.

The foregoing can perhaps be more easily visualized by reference to thedrawing, which corresponds to FIG- URE l of the above-mentioned patent.It shows a diagrammatic cross-section view of a heavy-oil-containingsand stratum 10 with an embodiment of our invention in operation thereinduring a period of steam injection. Stratum 10 is penetrated by a wellconventionally equipped with a surface casing 16, a production casing 21cemented through the stratum, and a tubing 25 extending to a screen 26surrounded by a gravel pack 27 below the base of the stratum.Perforations 24 near the top of stratum 10 extend from the annulusoutside tubing 25 through casing 21 and cement 22 into the body ofstratum 10.

During an injection period, steam from a boiler 35 at the ground surface13 flows down the annulus and out through perforations 24 into stratum10. Fluids entering the well through pack 27 and screen 26 are producedthrough the tubing 25, using a pump 29 as required, or if necessarymaintaining back pressure by a valve 38 to achieve desired flow rates.

The figure shows the stratum and well at an intermediate stage of steaminjection and gravity-drainage production. A drained zone 42 surroundsthe well in the upper part of the stratum, the heated oil and thecondensate of the steam which heated it having been produced throughtubing 25. The radial arrows in zone 42 generally depict the steam fiowfrom perforations 24 to the melting and gravity drainage zone 43, whichsurrounds zone 42 and separates it from the unheated part of stratum 10.

Throughout zone 42 the temperature and pressure are relatively uniform.They may be, for example, about 400 degrees F. and 250 pounds per squareinch, absolute, which correspond to the temperature and pressure ofsaturated steam. To understand why the temperature and pressure of zone42 must be uniform, assume for a moment that at some point in the zonethe temperature is less than 400 degrees F. The steam being presenteverywhere will immediately condense at this point until the heatreleased by the condensation brings it up to 400 degrees F. Then thecondensation will cease.

Because of the nature of the heavy oil that it is substantiallynon-llowable at the normal reservoir temperature, the outer face of themelting zone 43 acts like the wall of a container or pressure vessel.That is, the 250 pounds per square inch pressure of the steam in zone 42tends to force the heated hydrocarbons to flow radially away from thewell. It is unable to do so, however, because the hydrocarbons congealand plug the pore-space capillaries as soon as they contact the unheatedformation. Thus, the outer surface of the melting zone 43 is wherealmost the entire pressure differential occurs between the 250 poundsper square inch of zone 42 and the low natural pressure of the stratum10.

The temperature difl'crential between the 400 degrees F. of zone 42 andthe 50 degrees F., more or less (which is typical of the Athabasca tar)in the unheated formation outside of zone 43, is less abrupt. That is,most of the 350 degrees F. temperature drop occurs across the thicknessof zone 42, although there is some warming of the formation immediatelyoutside of the zone 43 before plastic flow of the tar and plugging ofthe pore space, which characterize the outer boundary of the zone, takeplace. Under the force of gravity, which acts completely independentlyof the force of the steam pressure, steam condensate and meltedhydrocarbon at 400 degrees F. (and of nearly the same specific gravity)flow downwardly and inwardly toward the well most rapidly on the innerface of melting zone 43. Between the inner and outer faces of zone 43,the downward flow of the draining liquids is progressively slower, asthe temperature decreases proceeding across the zone outwardly. Thearrows in zone 43 show the general direction of liquid flow duringgravity drainage.

The flow of heat by conduction through the zone 43, together with thegravity drainage of liquids downwardly and into the well borc,progressively exposes the unheated part of stratum to the heat of thesteam in zone 42, so that zone 42 enlarges, as zone 43 propagatesradially outwardly from the well. This propagation is most rapid nearthe top of stratum 10 where the drainage is most rapid because zone 43is there more nearly vertical.

Eventually a condition is reached where a large body of heated oil andcondensate is present below and surrounding the base of zone 42, but therate of its entry into the well bore is limited by the still substantialviscosity of the oil and the small force of gravity. It is at this timethat the improvement forming the present invention comes into operation.Steam injection through perforations 24 is stopped, valve 38 is openedwide, and pump 29 is operated to reduce the pressure inside screen 26rapidly to as low a value as possible.

Now, consider what happens to a droplet of steam condensate in the zone43 at a representative point 44. It is initially at, say 350 degrees F.and 250 pounds per square inch, absolute, pressure. Due to thetemperature drop across zone 43, its temperature is somewhat less thanthe maximum 400 degrees F. of the steam in zone 42. As liquid is drawninto the well bore, the pressure at 44 drops rapidly. When it becomesless than about pounds per square inch, absolute, the droplet ofcondensate at 350 degress F. and in contact with sand and other liquidsat the same temperature, can no longer remain a liquid but returns tovapor form, extracting its heat of vaporization from the heated solidsand liquids surrounding it. With continuing pressure reduction, itexpands as a gas and drives heated oil ahead of it toward the well bore.

What happens at point 44 is representative of the revaporization ofsteam condensate everywhere in the zone 43, as the pressure at eachpoint drops below that corresponding to saturated steam for thetemperature existing at that point within the zone. It is the relativelylarge force of this revaporized condensate within the body ofaccumulated liquids, compared with the smaller force of gravitydrainage, that so effectively supplements the latter in the recoveryprocess.

It is a matter of substantial importance in the application of thepresent invention to choose the proper time for cut-off of steaminjection and reduction of the pressure at the well bore. If the steaminjection is stopped and the pressure at the well bore is reduced tooearly in the life of a recovery project at the well, only a relativelysmall part of the oil in-place will be recovered during thepressuredrive stage because too small a volume of the reservoir has beenheated and the condensate is close to the well where it is immediatelyproduced without driving a quantity of oil ahead of it as occurs withlate depressuring. On the other hand, if the cut-off of steam injectionand reduction of well-bore pressure are delayed until a large body ofcondensate and heated oil accumulates and is ready to be produced intothe Well bore, reduction of the pressure at the well bore thenaccomplishes the rapid production of a large fraction of this oil.

In further substantiation of this aspect of our invention, the followingexperiments are cited. A model apparatus was constructed somewhatsimilar to that shown in FIG- URE 2 of the above-mentioned Patent2,881,838. The relative vertical and horizontal dimensions of the modelwere changed, however, so that the tar-sand body had a diameter of about15 inches and a height of 3 inches.

Given these dimensions and this dimensional ratio, by choice of theproper sizes of sand particles, the model approximated a volume of theAthabasca tar sand 50 feet thick and 250 feet in diameter. From aconsideration of the theoretical equations governing the flow of heatand the gravity drainage of liquids in porous media, it was calculatedthat 1 minute of operating time of the model was equivalent to about 28days of operation in the field. A flow of 1 cubic centimeter per minutein the model was calculated to correspond to about 1.8 barrels per dayof fluids produced in the field. For each of several test runs, themodel was filled with a mixture of Athabasca tar and sized sandparticles in substantially the ratio of their natural occurrence.Connate water was also present in substantially its naturalconcentration, but compared to the steam condensate its volume andeffect were negligible.

With the model at room temperature, operation of each run was initiatedby admitting into the perforated annulus around the central tube of themodel, 400 F. saturated steam at a gauge pressure of about 235 poundsper square inch. Liquid was removed from the central tube as fast as itcollected there by draining directly into a product vessel. A backpressure was held on the liquid-product receiver connected to the modelto maintain the entire system under the desired pressure of 235 poundsper square inch except during the intervals when the model wasdepressured. At these times the input steam was shut oil, and theproduct recovery was transferred to a receiver at atmospheric pressure.When the pressure in the model reached atmospheric, the depressuring wasconsidered to be completed, and steam injection was immediately resumed.Each experimental run of the model was considered ended when thetemperature at the top of the outside edge of the bed had risen F. aboveits initial room-temperature value, steam injection then beingcompletely stopped. This was equivalent to the arrival of heat at aradial distance of 125 feet from the well modeled.

In the accompanying Table I are shown the basic data obtained in fourconsecutive, directly comparable runs with this model. As will beimmediately apparent from an inspection of this table, Runs 13, 14 and16 produced rather similar results, both as regards theweight-percentage of recovery of the tar present in the model and asregards the water/ tar ratio, which is a measure of the amount of steaminjection required to obtain this production. In all of these tests, theamount of connate water present in the tar was negligible compared tothe amount of condensate water produced by the introduced steam.

Table I.-Model data The results of Run 15, however, are markedlydiiferent, as is evident from the fact that the percentage-recovery isalmost two and one-half times that of the average of the other threeruns, whereas the amount of steam injected and condensate produced isvery little different. This increase in tar production gives an over-allwater/tar ratio of about 2.9 instead of an average of around 7.3 andthus represents a marked increase in cfiiciency of utilization of theinjected steam.

The reason for this marked change in efficiency of operation is broughtout in Table II showing the depressuring data applicable to these modelruns. As is clear from this table, no depressuring of any sort was doneduring either of Runs 13 and 16, so that the sole recovery mechanism inoperation for these runs was gravity drainage as described in theaforementioned patent. In the cases of Runs 14 and 15, however, one ormore depressuring steps were carried out during the run, which stepscomprised cutting off the input steam by closing an inlet valve anddirecting the fluids produced from the model into a receiver atatmospheric rather than elevated pressure until the pressure in themodel vessel reached atmospheric.

is roughly proportional to the time involved, and no appreciable changein water-to-tar ratio was observed during the interval. It appears thatthere was no substantial body of heated oil and condensate formed andready to be forced into the well bore.

In the case of Run 15, the early depressuring steps was carried outsomewhat later in the duration of the run, namely. between 10 and 11.75minutes after the start. Thus, this step was performed about midway inthe duration of the run at a time when heating had probably occurred outfrom the well as a distance about equal to the formation thickness.Although not a significantly greater amount of tar was produced duringthis interval than in the depressing step of Run 14, it is apparent fromthe reduced water-to-tar ratio of 2.5 that there was some substantialbenefit to carrying out the depressuring step at this time. In otherwords, there was an appreciable body of heated oil ready to be driven tothe well bore by flashing condensate.

As compared with the second or later depressuring step, which was begunjust after the 25 temperature rise had occurred at the outer edge of themodel, the effect of this early depressuring step is relatively small.By starting the depressuring of the model at a time relatively late inthe operation when there is a quite large body of heated tar andcondensate in the formation, as is indicated by the occurrence ofheating at a distance nearly two and one-half times the bed thicknessfrom the Well bore, more tar is produced during the late depressuringstep than in all the rest of the operation put together. Furthermore,tar and condensate are produced in almost equal quantities rather thanin the ratio of l to 7 characteristics of gravity drainage. Thus, theover-all eticct of the late depressuring step is to more than double theamount of tar recovery for a given amount of input energy in the form ofsteam at elevated temperature and pressure. In summary, the tooearlydepressuring step of Run 14 is clearly below the lower time limit bywhich the present invention may be defined. The first depressuring stepof Run 15 appears to be close to but above the minimum time duration ofthe steam-injecting step, when some of the benefits of the invention maybe obtained. In other words, the initial period of steam injectionshould be at least about half as long as the time required for heat topropagate through a radial distance equal to 2.5 times the formationthickness. Preferably, it should be even longer, namely, about equal tothe time for radial heat propagation out to a distance 2.0 to 2.5 timesthe formation thickness.

From temperature observations at various points in the model during theforegoing and other runs, it was found that the radial propagation ofthe tar-melting face at the top of the formation was approximatelylinear with time.

Run No 13 14 15 16 Early dcpressuring interval Not done Between 4 and 9iuiu- Between 10 and 11.75 Not done.

ntes after start. minutes after start.

Percent of produced tar obtained during early 18 percent 23.2 percentdepressuring.

Water/tar ratio during this interval 7.8 A 2.5

Late depressnring interval Not done Not done. Between lisjfi and 21 Notdone.

minutes after start. Ilercent of produced tor obtained during late 57.3percent depressuring. Water/tar ratio during this interval 1.1

As will be seen from Table I, depressuring was used in different ways inRuns 14 and 15. From these runs it is possible to see the effect of adepressuring step carried out very early in the duration of a run ascompared with carrying it out in the middle or much later in the run.Thus, in Run 14 only one depressuring step was performed. between thetimes of 4 and 9 minutes after the start of the run. This depressuringwas obviously too early to have any appreciable effect on the results ofthe run, as the 18 percent of produced tar obtained during this timeinterval Both in the middle and at the end of each run, it was notedthat the tar produced by gravity drainage was about 10 percent of theoriginal tar in place within and underlying the heated zone.

This aiTords a very convenient way of estimating the radial propagationof the tar-melting face during a field operation. Knowing the volume oftar in place in a cylindrical volume of radius r around a well bore in aformation of known thickness, as can be determined by computation frommeasurements on cores, when 10 percent of this volume of tar has beenrecovered by gravity drainage, then the tar-melting face will havepropagated a distance r from the well bore. The minimum time durationfor the initial steam-injection step can thus be defined as extendinguntil about 10 percent of the in-place oil within a cylindrical volumeof radius equal to the forma tion thickness has been produced by gravitydrainage. The preferred time for depressuring is when about 10 percentof the in-place oil within a cylindrical volume having a radius 2 to 2.5times the formation thickness has been produced. Subsequent pressuringintervals in a series of alternate pressuring and depressuring stepsshould be of similar length, until there is substantial depletion,and/or heat losses to the overburden become prohibitive.

In terms of field operations, the above test results can be scaled up inaccordance with the factors previously stated. The calculated fieldresults from such scaling up of the model data are shown in Table III.Thus, the average production rate of 66.7 barrels of tar per day for Run15 is more than double the best average production rate of 27.1 barrelsof tar per day for Run 13. These are the calculated results obtainablefrom a tar formation 50 feet thick, heated out to a radius of 125 feetfrom the well bore. The over-all water-to-tar ratios for theseproduction figures are the same as for the model and thus likewisedemonstrate a marked improvement in utilization of the energy of theinput steam.

Table lII.-Calculated field results from scaled-up model While theforegoing results of the model runs and calculated results of fieldoperation are presented quantitatively, they are to be considered asonly qualitative as regards actual field operation. The scaling factorsbetween the model and the field operations are strictly applicable onlyto the case of gravity drainage (i.e., to Runs 13 and 16), and even forthat case with a precision which is not completely known. While adepressuring step in field operations, performed at a time when thetar-melting face has propagated a distance from the well bore equal totwo to two and one-half times the bed thickness, can certainly beexpected to improve the efficiency of the operation by a substantialfactor, it will probably be different from the 2.5 factor characterizingthese model tests. Nevertheless, it is clear that the force of gravitydrainage can be greatly supplemented in the manner set forth in theabove description.

Although the model runs were terminated when heat had been carriedradially by the steam out to about 2.5 times the stratum thickness, afield operation would probably not have reached its economic limit ofrecovery at a corresponding time. Thus, at the conclusion of productioninduced by the depressuring step designated as late depressuring above,the entire pressuring and depressuring cycle will be repeated one ormore additional times. That is, steam injection at an elevatedtemperature and pressure will be resumed and continued until there isanother large body of condensate and melted tar present in theformation. Depressuring will then bring this body of liquid rapidly tothe well bore for recovery. As a result of the higher producing ratesand greater steam economy accompanying this alternate pressuring anddepressuring of the formation, a larger percentage of in-place tar oroil can be recovered before heat losses become prohibitive than can berecovered when gravity drainage alone is relied upon to move the liquidsto the producing well.

While we have described our invention in terms of the foregoing specificexamples and details, it is to be understood that other and furthermodifications of the procedure may be made in particular instances. Thescope of the invention, therefore, should not be considered as limitedto the details set forth, but it is properly to be ascertained from theappended claims.

We claim:

1. In a method of recovering, from an underground stratum in which itoccurs, heavy oil which is substantially non-fiOWable by the applicationof driving-fluid pressure thereto and which retains substantialviscosity at elevated temperatures, said stratum being substantiallycompletely penetrated by a well extending from the ground surface, whichmethod comprises the steps of injecting substantially only steam at anelevated temperature and pressure into said well and thence into saidstratum to heat by condensation substantially the entire stratum faceexposed in said well and reduce the viscosity of the oil at said face,whereby heated oil and steam condensate flow downwardly by gravitydrainage toward the bottom of said well and continuously expose unheatedoil and formation behind said face, and withdrawing said heated oil andcondensate from near the bottom of said well at substantially the ratethey collect there while holding sufficient back pressure on said wellto maintain said pressure and temperature at their elevated valueswithin the heated volume of said stratum, whereby a zone of melting oiland gravity drainage of heated oil and steam condensate propagatesradially outwardly from said well through at least the upper part ofsaid stratum, the improvement which comprises performing said method asa series of alternating pressuring and depressuring steps in which eachpressuring step comprises continuing said steam-injecting, withdrawing,and back-pressure-holding steps until there is an accumulation of heatedheavy oil and steam condensate of substantial size in said stratum, andeach depressuring step comprises releasing said back pressure andrapidly withdrawing fluids from near the bottom of said well to reducethe pressure at said well bore to a low value and thereby establishthroughout said heated volume a substantial pressure gradient to aid theforce of gravity drainage in moving said accumulation toward said wellbore.

2. In a method of recovering, from an underground stratum in which itoccurs, heavy oil which is substantially non-fiowable by the applicationof driving-fluid pressure thereto and which retains substantialviscosity at elevated temperatures, said stratum being substantiallycompletely penetrated by a well extending from the ground surface, whichmethod comprises the steps of injecting substantially only steam at anelevated temperature and pressure into said well and thence into saidstratum to heat by condensation substantially the entire stratum faceexposed in said well and reduce the viscosity of the oil at said face,whereby heated oil and steam condensate flow downwardly by gravitydrainage toward the bottom of said Well and continuously expose unheatedoil and formation behind said face, and withdrawing said heated oil andcondensate from near the bottom of said well at substantially the ratethey collect there while holding sufficient back pressure on said wellto maintain said pressure and temperature at their elevated valueswithin the heated volume of said stratum, whereby a zone of melting oiland gravity drainage of heated oil and steam condensate propagatesradially outwardly from said well through at least the upper part ofsaid stratum, the improvement which comprises performing said method asa series of alternating pressuring and depressuring steps, all of saidpressuring steps being of approximately equal time duration, each ofsaid pressuring steps comprising continuing said steam-injecting,withdrawing, and back-pressure-holding steps for a period of time atleast as long as that required to produce by gravity drainage 10 percentof the in-place heavy oil in a cylindrical volume of said stratumsurrounding said well of a radius equal to the stratum thickness, andeach depressuring step comprising releasing said back-pressure andrapidly withdrawing fiuids from near the bottom of said well to reducethe pressure at said well bore to a low value and thereby establishthroughout said heated volume a substantial pressure gradient to aid theforce of gravity drainage in moving said accumulation toward said wellbore.

3. In a method of recovering, from an underground stratum in which itoccurs, heavy oil which is substantially non-llowable by the applicationof driving-fluid pressure thereto and which retains substantialviscosity at elevated temperatures, said stratum being substantiallycomplctely penetrated by a well extending from the ground surface, whichmethod comprises the steps of injecting substantially only steam at anelevated temperature and pressure into said well and thence into saidstratum to heat by condensation substantially the entire stratum faceexposed in said well and reduce the viscosity of the oil at said face,whereby heated oil and steam condensate flow downwardly by gravitydrainage toward the bottom of said well and continuously expose unheatedoil and formation behind said face, and withdrawing said heated oil andcondensate from near the bottom of said well at substantially the ratethey collect there while holding suihcient back pressure on said well tomaintain said pressure and temperature at their elevated values withinthe heated volume of said stratum, whereby a zone of melting oil andgravity drainage of heated oil and steam condensate propagates radiallyoutwardly from said well through at least the upper part of saidstratum, the im provement which comprises performing said method as aseries of alternating pressuring and depressuring steps, all of saidpressuring steps being of approximately equal time duration, each ofsaid pressuring steps comprising continuing said steam-injecting,Withdrawing, and backpressure-holding steps for a period of time equalto that required to produce by gravity drainage 10 percent of thein-place heavy oil in a cylindrical volume surrounding said well and ofa radius between about 2.0 and 2.5 times the stratum thickness, and eachdepressuring step comprising releasing said back pressure and rapidlywithdrawing fluids from near the bottom of said well to reduce thepressure at said well bore to a low value and thereby establishthroughout said heated volume a substantial pressure gradient to aid theforce of gravity drainage in moving said accumulation toward said wellbore.

4. In a method of recovering, from an underground stratum in which itoccurs, heavy oil which is substantialy non-flowable by the applicationof drivingfiuid pressure thereto and which retains substantial viscosityat elevated temperatures, said stratum being substantially completelypenetrated by a well extending from the ground surface, which methodcomprises the steps of injecting substantially only steam at an elevatedtemperature and pressure into said well and thence into said stratum toheat by condensation substantially the entire stratum face exposed insaid well and reduce the viscosity of the oil at said face, wherebyheated oil and steam condensate flow downwardly by gravity drainagetoward the bottom of said well and continuously expose unheated oil andformation behind said face, and withdrawing said heated oil andcondensate from near the bottom of said well at substantially the ratethey collect there while hold ing sufiicient back pressure on said wellto maintain said pressure and temperature at their elevated valueswithin the heated volume of said stratum, whereby a zone of melting oiland gravity drainage of heated oil and steam condensate propagatesradially outwardly from said well through at least the upper part ofsaid stratum, the improvement which comprises the steps of discontinuingsaid steaminjccting, withdrawing and back-pressure-holding steps at atime when there is an accumulation of heated heavy oil and steamcondensate of substantial size within said stratum. and rapidlywithdrawing fluids from near the bottom of said well to reduce thepressure at said well bore to a low value and thereby establishthroughout said heated volume a substantial pressure gradient to aid theforce of gravity drainage in moving said accumulation toward said wellbore.

5'. In a method of recovering, from an underground stratum in which itoccurs, heavy oil which is substantialiy non-fiowable by the applicationof driving-fluid pressure thereto and which retains substantialviscosity at elevated temperatures, said stratum being substantiallycompletely penetrated by a well extending from the ground surface, whichmethod comprises the steps of injecting substantially only steam at anelevated temperature and pressure into said well and thence into saidstratum to heat by condensation substantially the entire stratum faceexposed in said well and reduce the viscosity of the oil at said face,whereby heated oil and steam condensate tlow downwardly by gravitydrainage toward the bottom of said well and continuously expose unheatedoil and formation behind said face, and withdrawing said heated oil andcondensate from near the bottom of said well at substantially the ratethey collect there while holding suflicient back pressure on said wellto maintain said pressure and temperature at their elevated valueswithin the heated volume of said stratum, whereby a zone of melting oiland gravity drainage of heated oil and steam condensate propagatesradially outwardly from said well through at least the upper part ofsaid stratum, the improvement which comprises the steps of discontinuingsaid steam-injecting, withdrawing, and back pressure-holding steps at atime when said zone has propagated radially from said well a distancewhich is at least as great as the stratum thickness, and rapidlywithdrawing fiuids from near the bottom of said well to reduce thepressure at said well bore to a low value and thereby establishthroughout said heated volume a substantial pressure gradient to aid theforce of gravity drainage in moving said accumulation toward said wellbore.

6. In a method of recovering, from an underground stratum in which itoccurs, heavy oil which is substantially non-fiowable by the applicationof driving-fluid pressure thereto and which retains substantialviscosity at elevated temperatures, said stratum being substantiallycompletely penetrated by a well extending from the ground surface, whichmethod comprises the steps of injecting substantially only steam at anelevated temperature and pressure into said well and thence into saidstratum to heat by condensation substantially the entire stratum faceexposed in said well and reduce the viscosity of the oil at said face,whereby heated oil and steam condensate flow downwardly by gravitydrainage toward the bottom of said well and continuously expose unheatedoil and formation behind said face, and withdrawing said heated oil andcondensate from near the bottom of said well at substantially the ratethey collect there while holding sufficient back pressure on said wellto maintain said pressure and temperature at their elevated valueswithin the heated volume of said stratum, whereby a zone of melting oiland gravity drainage of heated oil and steam condensate propagatesradially outwardly from said well through at least the upper part ofsaid stratum, the improvement which comprises the steps of discontinuingsaid steam-injecting, withdrawing, and back-pressureholding steps at atime when said zone has propagated radially from said well a distancebetween about 2.0 and 25 times the stratum thickness, and rapidlywithdrawing fluids from near the bottom of said well to reduce thepressure .at said well bore to a low value and thereby establishthroughout said heated volume a substantial pressure gradient to aid theforce of gravity drainage in moving said accumulation toward said wellbore.

7. In a method of recovering, from an underground stratum in which itoccurs, heavy oil which is substantially non-tlowable by the applicationof driving-fluid pressure thereto and which retains substantialviscosity at elevated temperatures, said stratum being substantiallycompletely penetrated by a well extending from the ground surface, whichmethod comprises the steps of injecting substantially only steam at anelevated temperature and pressure into said well and thence into saidstratum to heat by condensation substantially the entire stratum faceexposed in said well and reduce the viscosity of the oil at said face,whereby heated oil and steam condensate llow downwardly by gravitydrainage toward the bottom of said well and continuously exposedunheated oil and formation behind said face, and withdrawirlg saidheated oil and condensate from near the bottom of said well atsubstantially the rate they collect there while holding sufiicient backpressure on said well to maintain said pressure and temperature at theirelevated values within the heated volume of said stratum, whereby a zoneof melting oil and gravity drainage of heated oil and steam condensatepropagates radially outwardly from said well through at least the upperpart of said stratum, the improvement which comprises the steps ofdiscontinuing said steam-injecting, withdrawing, andback-pressure-holding steps at a time when at least 10 percent of thein-place heavy oil in a cylindrical volume of said stratum surroundingsaid well and of a radius equal to the stratum thickness has beenproduced by gravity drainage, and rapidly withdrawing fluids from nearthe bottom of said well to reduce the pressure at said well bore to alow value and thereby establish throughout said heated volume asubstantial pressure gradient to aid the force of gravity drainage inmoving said accumulation toward said well bore.

8. In a method of recovering, from an underground stratum in which itoccurs, heavy oil which is substantially non-flowable by the applicationof driving-fluid pressure thereto and which retains substantialviscosity at elevated temperatures, said stratum being substantiallycompletely penetrated by a well extending from the ground surface, whichmethod comprises the steps of injecting substantially only steam at anelevated temperature and pressure into said well and thence into saidstratum to heat by condensation substantially the entire stratum faceexposed in said well and reduce the viscosity of the oil at said face,whereby heated oil and steam condensate flow downwardly by gravitydrainage toward the bottom of said well and continuously expose unheatedoil and formation behind said face, and withdrawing said heated oil andcondensate from near the bottom of said well at substantially the ratethey collect there while holding suflicient back pressure on said wellto maintain said pressure and temperature at their elevated valueswithin the heated volume of said stratum, whereby a zone of melting oiland gravity drainage of heated oil and steam condensate propagatesradially outwardly from said well through at least the upper part ofsaid stratum, the improvement which comprises the steps of discontinuingsaid steam-injecting, withdrawing, and back-pressureholding steps at atime when 10 percent of the in-place heavy oil in a cylindrical volumeof said stratum surrounding said well and of a radius between about 2.0and 2.5 times the stratum thickness has been produced by gravitydrainage, and rapidly withdrawing fluids from near the bottom of saidwell to reduce the pressure at said well bore to a low value and therebyestablish through said heated volume a substantial pressure gradient toaid the force of gravity drainage in moving said accumulation towardsaid well bore.

9. In a method of recovering, from an underground stratum in which itoccurs, heavy oil, said stratum being penetrated by a well extendingfrom the ground surface, which method comprises the steps of injectingsteam at an elevated temperature and pressure into said well and thenceinto said stratum to heat by condensation subsfanriully the entirestratum jruic exposed in said well and reduce the viscosity of the oila! said face, whereby healed oil and steam condensate flow downwardly bygravity drainage toward the bottom of said wcll and continuously exposeunheated oil and formation behind said face, and withdrawing said heatedoil and condensate from near the bottom of said well at substantiallythe rate they collect there while holding su/ficicnt back pressure onsaid well to maintain said pressure and temperature or their elevatedvalues within the heated volume of said stratum, whereby a zone ofheated oil and gravity drainage of said oil and steam condensatepropagates radially outwardly from said well through at least the upperpart of said stratum, the improvement which comprises performing soldmethod as a series of alternating pressuring and de t-assuring steps inwhich each pressuring step comprises continuing injection of steam,withdrawing, and buck-pressure-holding steps until there is anaccumulation of heated heavy oil and steam condensate of substantialsize in said stratum, and each dcprcssuring step compriscs releasingsaid back pressure and rapidly withdrawing fluids from near the bottomof said well to reduce the pressure at said well bore to a low value andthereby establish throughout said heated volume a substantial pressuregradient to aid the force of gravity drainage in moving saidaccumulation toward said well bore.

10. In a method of recovering, from an underground stratum in which itoccurs, heavy oil, sold stratum being penetrated by a well extendingfrom the ground surface, which method comprises the steps of injectingsteam at an elevated temperature and pressure into said wcll and thenceinto said stratum to heat by condensation substantially the entirestratum face exposed in said well and reduce the viscosity of the oil atsaid face, whereby heated oil and steam condensate flow downwardly bygravity drainage toward the bottom of said well and continuously exposeunheated oil and formation behind said face, and withdrawing said heatedoil and condensate from near the bottom of said well at substantiallythe rate they collect there while holding sufiicienr back pressure onsaid well to maintain said pressure and temperature at their elevatedvalues within the heated volume of said stratum, whereby a zone ofheated oil and gravity drainage of said healed oil and steam condensatepropagrucs radially outwardly from said well through at least the upperpart of said stratum, the improvement which comprises the steps ofdiscontinuing said steam-injecting, withdrawing, andbucltprcss1lrc-l:olding stcps at a time when said zone has propagatedradially from said well a distance which is or least as great as thestratum thickness, and rapidly withdrawing fluids from near the bottomof said well to reduce the pressure at said well bore to a low value andthereby causing part of said condensate to be vaporized by the heatstored in the liquids and in the reservoir rock itself so that someflashing of said condensate into steam occurs, thus adding to the forceof gravity drainage in moving said accumulation toward said well bore.

References Cited by the Examiner The following references, cited by theExaminer, are of record in the patented lile of this patent or theoriginal patent.

CHARLES E. OCONNELL, Primary Examiner.

